This invention relates to improved pattern-forming compositions, and more particularly to improved pattern-forming thermoplastic compositions for use in investment casting processes, and to improved investment casting patterns and processes using such compositions.
Various investment casting processes, also called lost wax processes, have been known for centuries. Through the ages, compositions for the construction of disposable patterns used in such processes have been selected for several characteristics, including such important properties as dimensional reproducibility and the ability to produce a highly accurate surface finish in the molded disposable pattern. Because such properties are critical to many products manufactured by lost wax processes, repeated efforts have been and are being made to improve such properties of pattern-forming compositions.
The quality and properties of an investment casting depend inextricably upon the quality of the disposable pattern, which, in turn, depends upon the characteristics of the pattern-forming compositions of which the disposable patterns are molded.
Disposable thermoplastic patterns are usually formed by heating and melting a thermoplastic composition which is adapted to form a pattern, introducing the molten composition into a mold, and then cooling the composition until it solidifies to form a disposable pattern. Thereafter, the disposable thermoplastic pattern is removed from the mold, assembled with the other patterns, if necessary, and then encased in a mold forming material, usually a ceramic material, in accordance with one of a variety of known methods, thereby forming a shell or cast about the disposable pattern.
Next, a major portion of the disposable pattern is removed by melting at a moderately elevated temperature by autoclaving, and substantially all of the remainder of the pattern material is removed at a substantially higher temperature by vaporization or burning or both so that, except for any ash residue from the pattern material, the inner surface of the shell or mold is clean. The shell or mold is then ready for one-time use for forming an investment cast part. A text describing known procedures used in lost wax processes is entitled Investment Casting, H. T. Bidwell, Machinery Publishing Co., Ltd., England, 1969.
By this process, the surface characteristics of the disposable pattern and of the ceramic shell are "transferred" to the final casting. Thus, the above-discussed properties of the pattern-forming composition and any residue therefrom will affect the surface characteristics and metallurgical characteristics of a casting. Similarly, variations in expansion and contraction of compositions from which disposable patterns are formed result in shells or casts of varying dimensions, and therefore inconsistent castings. Accordingly, the properties of pattern-forming compositions are critical to the investment casting.
Therefore, a satisfactory composition comprising thermoplastic pattern material should not only resist change in dimensions with changes in temperature, but also leave little or no ash residue after burning so that an accurate casting may be made. An ash residue which is essentially stable at burning temperatures (e.g., 1800.degree. F.) and which volatilizes at the temperature of molten steel (e.g., 3000.degree. F.) is particularly harmful since it tends to leave gas bubble irregularities on the surface of the cast product. Therefore, it is desired that the composition leave no more than 0.1%, and preferably no more than 0.01%, by weight of ash when burned at 1800.degree. F.
When the composition comprising thermoplastic pattern material is heated, it is also desirable that the material not decompose before melting, and that it be capable of flowing freely when thoroughly melted. A thermoplastic composition that melts to a free flowing state can be substantially emptied from a mold in the molten state, thereby minimizing the burden on the subsequent burning operation and forming a minimum of objectional fumes on burning. It is thus desired that the thermoplastic material have a viscosity at 248.degree. F. (120.degree. C.) of no greater than 5000 centipoise. At 200.degree. F. (93.degree. C.), the viscosity of the thermoplastic material should have a viscosity of no greater than about 20,000 cps, preferably no greater than about 10,000 cps.
Many thermoplastic pattern-forming compositions have been used or suggested for use in the past. As the name "lost wax" process implies, waxes, such as natural waxes, including beeswax and the like, were originally used as thermoplastic pattern materials. As other pattern materials were sought to improve the properties of disposable patterns, other natural thermoplastics, such as gum damar, gum rosin, esparto waxes, and the like, mineral waxes, such as those extracted from soft coal, and the like, and petroleum waxes were adopted for use.
As a result of this search, modified waxes, such as microcrystalline waxes, were developed for use in lost wax processes. More recently, as a result of the continuing efforts of researchers to improve upon and develop new thermoplastic materials, synthetic thermoplastics have been used as pattern materials or as thermoplastic pattern forming composition modifiers. Those efforts have also resulted in the use by some investment casters of materials other than thermoplastic pattern materials, such as mixtures of metallic salts and mercury. Moreover, pattern compositions that flow out of a mold more quickly and completely than conventional compositions have been sought.
Other efforts to increase the dimensional accuracy and stability of thermoplastic pattern-forming compositions involve the addition of solid filler materials. Accordingly, thermoplastic polystyrene powder, especially polystyrene cross-linked with divinylbenzene, and urea powder have been included in minor quantities in thermoplastic pattern-forming compositions as filler material. A "filler" is a solid particulate that is included as an inert additive in the sense that it does not react chemically with the thermoplastic through which it is dispersed. The filler remains a separate phase and retains its solid particulate identity throughout the investment casting process. Organic acids, such as fumaric acid, adipic acid and isophthalic acid, have also sometimes been used as solid fillers. Usually the filler is included in amounts of up to about 40% by volume of the thermoplastic pattern-forming composition, and in a particle size generally in the range of about 175 to 250 mesh. Thus, for a typical filler, at least about 90%, preferably 100%, by weight of the particles may pass through a 100 mesh sieve and at least about 50%, preferably about 50%, by weight of the particles pass through a 200 mesh sieve.
However, these fillers suffer several drawbacks. For example, conventional fillers commonly contain an organic component that produces an ash residue at the elevated temperatures involved in the investment casting process. Thus, as an illustration, polystyrene contains a carbonaceous benzene ring which may decompose to form carbon in addition to hydrogen gas. The ash residue, i.e., the impurity, thereby introduced into the mold limits accurate dimensional reproducibility and the ability to produce a highly accurate surface finish in the molded disposable pattern. Thus, standard fillers have been found to leave undesirably high amounts of ash residue in the mold.
Various fillers also involve several other problems. For example, urea tends to decompose when wax is melted, thermoplastic styrene can melt if overheated and organic acids have high specific gravities, and so tend to settle quickly when not sufficiently agitated. In addition, many fillers have a relatively high thermal conductivity, which can lead to premature expansion of the composition upon autoclaving, thereby causing shell cracking.
Thus, fillers for pattern-forming materials are still needed that aid the composition in flowing out of a mold more quickly and completely, that leave less ash residue in the mold, and that result in patterns with smooth surfaces and less shrinkage.